fix: reuse already attached attachments from spread attributes

.changeset/rotten-lizards-remember.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: reuse already attached attachments from spread attributes

packages/svelte/src/internal/client/dom/elements/attributes.js

@@ -448,7 +448,7 @@ export function set_attributes(element, prev, next, css_hash, skip_warning = fal
 	}
 
 	for (let symbol of Object.getOwnPropertySymbols(next)) {
-		if (symbol.description === ATTACHMENT_KEY) {
+		if (symbol.description === ATTACHMENT_KEY && !(key in prev)) {
 			attach(element, () => next[symbol]);
 		}
 	}